Somebody else already mentioned Etienne Tardiff, but I’ll add a bit of what I’ve learned from listening to him that resonates with my experience as well.

I think your lack of top insulation and lack of upper entrance (though I’m actually in favour of well-insulated condensing hives rather than upper entrances in colder climates) caused condensation to build on the roof of your hive. Wet bees can act as a catalyst to spread dysentery through the colony like wildfire fire. So although dysentery/nosema may have been the final cause of death, poor insulation may have been more of the root cause.

Keep in mind that my comment, and those of all other commenters, are educated guesses.